802.11 -phydisi.unitn.it/locigno/didattica/nc/11-12/021_802.11-phy_s.pdf · frequency hopping...
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Nomadic Communications
802.11 - PHY
Renato Lo Cigno
LoCigno@disi.unitn.it - Tel: 2026
Dipartimento di Ingegneria e Scienza dell’Informazione
Home Page: http://isi.unitn.it/locigno/index.php/teaching-duties/nomadic-communications
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locigno@disi.unitn.it 2
CopyrightCopyright
Quest’opera è protetta dalla licenza:
Creative Commons
Attribuzione-Non commerciale-Non opere derivate
2.5 Italia License
Per i dettagli, consultarehttp://creativecommons.org/licenses/by-nc-nd/2.5/it/
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locigno@disi.unitn.it 3
Physical Layer
A collection of different access techniques:
� Infrared (IR), never really used
� Frequency hopping spread spectrum (FHSS), 1-2 Mbit/s now obsolete
� Direct sequence spread spectrum (DSSS), 1,2,5.5 and 11 Mbit/s, the most diffused till 3-4 years ago
� Orthogonal Frequency Division Multiplexing (OFDM), nothing to do with FDM, this is a modulation technique 6 to 54 Mbit/s now the most used, and beyond
� Four different standards: 802.11; /b; /a/h/g; /n
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locigno@disi.unitn.it 4
PHY layer subdivision
� PLCP: Physical Layer Convergence Protocol
� PMD: Physical Medium DependantMAC
PLCP
PMD
MPDU
PPDU
� PPDU contains the PHY layer headers stripped when the PDU is passed to the MAC
� PMD defines the specific electromagnetic characteristics used on different PHY means
� PLCP Header
� Is actually already dependent on the PMD
� Includes sync preambles and further info on the encoding of the remaining part of the MPDU
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locigno@disi.unitn.it 5
Infrared
• Works in the regular IR LED range, i.e. 850-950 nm
• Used indoor only
• Employes diffusive transmissions, nodes can receive
both scattered and line-of-sight signals
• Max output power: 2W
• Never really implemented ... tough can have
“reasons” in some environments, and is very cheap
• Tx uses a LED, Rx a Photodiode
• Wavelength between 850 and 950 nm
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locigno@disi.unitn.it 6
Infrared
• Modulation is “baseband” PPM (Pulse Position Modulation),
similar to on-off keying with Manchester encoding to ensure
constant sync transisions
• 1 Mbit/s: 16/4 PPM • 0000 � 0000000000000001• 0001 � 0000000000000010• 0010 � 0000000000000100• 0011 � 0000000000001000• 0100 � 0000000000010000• ...
• 2 Mbit/s: 4/2 PPM • 00 � 0001• 01 � 0010• 10 � 0100• 11 � 1000
• Pulses are 250 ns
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locigno@disi.unitn.it 7
IR PLCP frame
� SYNC: variable length, synchronization and optional fields on gain control and channel quality
� SFD (Start Frame Delimiter): 4 L-PPM slots with a hex symbol of 1001. This field indicates the start of the PLCP preample and performs bit and symbol synchronization
� DR (Data Rate): 3 L-PPM slots and indicates the speed used:
� 1 Mbps: 000; 2 Mbps: 001
� DCLA (DC Level Adjustment): used for DC level stabilization, 32 L-PPM slot and looks like this:
� 1 Mbps: 00000000100000000000000010000000
� 2 Mbps: 00100010001000100010001000100010
� LENGTH: number of octets transmitted in the PSDU: 16-bit integer
� CRC: header protection – 16 bits
� PSDU: actual data coming from the MAC layer; Max 2500 octets, Min 0
SYNC SFD DR DCLA LENGTH CRC PSDU
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locigno@disi.unitn.it 8
802.11 radios: Spread Spectrum
• All radio-based PHY layers employ SpreasSpectrum
• Frequency Hopping : transmit over random sequence of frequencies
• Direct Sequence: random sequence (known to both sender and receiver), called chipping code
• OFDM: spread the signal ove many subcarriers withFFT based techniques
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locigno@disi.unitn.it 9
802.11 radios: Power
� Power radiation is limited to
� 100mW EIRP in EU
� 100mW EIRP in USA
� 10mW EIRP in Japan
� NIC cards are the same all over the world: changing power is just a matter of firmware config.
� EIRP: Equivalent Isotropic Radiated Power
� In practice defines a power density on air and not a transmittedpower
� Using high gain antennas (in Tx) can be (legally) done only by reducing the transmitted power or to compensate for losses on cables/electronics
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locigno@disi.unitn.it 10
802.11 PHY evolution
40-250OFDM15,30,45,60,90,
120,135,150 (40 MHz); divide by 2 for 20 MHz
2.4GHz/
20/40MHz
n – 09
20-150OFDM6,9,12,18,24,36,48,542.4GHz/20MHzg – 03
20-150OFDM6,9,12,18,24,36,48,545.0GHz/20MHza/h – 99
25-150DSSS5.5,112.4GHz/20MHzb – 99
20-100FHSS1,22.4GHz/20MHz- --97
Max dist
in—out
SS technique
Data Rates (Mbit/s)Freq/Bandwst—year
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locigno@disi.unitn.it 11
Band allocations
� ISM: Industrial Scientific Medical
� Unlicenced bands for generic use
� Normally not used for communications (cfr Cellular, TV, Radio, ...)
� Law dictates limits in use, but do not guarantee interference-free operations
� Similar to radio-amateurs bands ... but for the fact that those are only for study and not for commercial use
� 2.4—2.5 GHz
� Actually 83.5 MHz of bandwidth in EU (13 channels) and 71.5 in US (11 channels)
� 4.9—5.9 GHz
� Actual bandwidth assigned depends on countries, in US and EU there are normally 20-25 channels (about 120-150 MHz of bandwidth)
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locigno@disi.unitn.it 12
2.4 GHz channels for 802.11 FHSS
� 79 1 MHz channels
� Limits Tx speed since Tx happens on one single channel at a time
1 2 3 77 78 79
1 MHz
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locigno@disi.unitn.it 13
2.4 GHz channels for 802.11b/g
� At most 3 independet (orthogonal) FDM channels
� 1,6,11; 1,7,12; 2,7,12; 1,7,13, ...
� Partially overlapping channels are noxious for CarrienrSensing � exposed and hidden terminals result
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locigno@disi.unitn.it 14
5 GHz channels for 802.11a
� Overlapping channels are avoided
� in US 12 non-overlapping channels centered at
� 5.180, 5.200, 5.220, 5.240, 5.260, 5.280, 5.300, 5.320
� 5.745, 5.765. 5.785, 5.805
� in EU the frequencies above are for hyperlan2 (licensed) thus intermediate frequencies are used
� 5.35—5.47 GHz 6 non overlapping channels
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locigno@disi.unitn.it 15
Global 5 GHz band plan
Original by Martin Johnsson: http://www.hiperlan2.com/presdocs/site/whitepaper.pdf
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locigno@disi.unitn.it 16
IEEE 802.11/b PHY
DSSS
1,2,5.5,11 Mbps
3 Indoor/Outdoor
2.4-2.4835 GHz
83.5 MHz
Sep. 1999
802.11b (Wi-Fi)
FHSS, DSSSPhysical layer
83.5 MHzBandwidth
2.4-2.4835 GHzFrequency of operation
1,2 MbpsData rate per channel
3 Indoor/OutdoorNumber of non-overlapping channels
July 1997Standard approval
802.11
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locigno@disi.unitn.it 17
802.11 - FHSS
• 1 or 2 Mbit/s only @ 2.4 GHz
• GFSK modulation: base waveforms are gaussianshaped, bits are encoded shifting frequency, butthe technique is such that it can also beinterpreted as
• BPSK (2GFSK � 1Mbit/s)
• QPSK (4GFSK � 2Mbit/s)
• Slow Frequency Hopping SS
• 20 to 400 ms dwell time ⇒ max 50 hop/s, min 2.5 hop/s
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locigno@disi.unitn.it 18
802.11 - FHSS
• 1 channel is used as guard
• 78 channels are divided into 3 orthogonal channels of 26 subchannels each
• Hopping is a PN sequence over the 26 channels
• Tx and Rx must agree on the hopping sequence
1 2 3 77 7876
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locigno@disi.unitn.it 19
FH PLCP frame
� Always transmitted at 1 Mbits/s
� SYNC: 80 bits alternating 01010101 . . .
� SFD: 16 bits (0000 1100 1011 1101)
� PLW: number of octets transmitted in the PSDU: 12-bit integer
� PSF: 4 bits, indicates the rate used in the PSDU
� CRC: header protection – 16 bits� Generating Polinomial G(x) = x16+x12+x5+1
� PSDU: actual data coming from the MAC layer; Max 4095 octets, Min 0 � Scrambled to “whiten” it
SYNC SFD PSFPLW HEC PSDU
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locigno@disi.unitn.it 20
Data scrambling (whitening)
� It is a simple feedback shift register generating a 127 bit long sequence XORed with data
� S(x) = x7+x4+1
� Every 32 bits a 33-rd is inserted to suppress eventual biases
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locigno@disi.unitn.it 21
DSSS PHY
� Direct Spreading through digital multiplication with a chip sequence
� The scope is fading protection and not CDMA
� Max 3 FDM orthogonal channels
� Different specifications for the 1-2 and 5.5-11 PHY speeds
� Different headers
� Long for 802.11 and 802.11b in compatibility mode
� Short for 802.11b High Rates only (5.5-11)
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locigno@disi.unitn.it 22
802.11b Long Preamble PLCP PDU
� Compatible with legacy IEEE 802.11 systems
� Preamble (SYNC + Start of Frame Delimiter) allows receiver to acquire the signal and synchronize itself with the transmitter
� Signal identifies the modulation scheme, transmission rate
� Length specifies the length of the MPDU (expressed in time to transmit it)
� CRC same as HEC of FHSS
SYNC SFD Signal Service Length CRC MPDU
128 16 8 8 16 16
PLCPPreamble1 Mbit/s
PLCP PDU (PPDU)
PLCP Header1 Mbit/s 1 – 2 – 5.5 – 11
Mbit/s
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locigno@disi.unitn.it 23
802.11b Short Preamble PLCP PDU
� Not compatible with legacy IEEE 802.11 systems
� Fields meaning is the same
SYNC SFD Signal Service Length CRC MPDU
58 16 8 8 16 16
PLCPPreamble1Mbit/s
PLCP PDU (PPDU)
PLCP header2Mbit/s 2 – 5.5 – 11 Mbit/s
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locigno@disi.unitn.it 24
Tx for 1-2 Mbit/s
� Spreading is obtained with an 11 bits Barker code
� +1, –1, +1, +1, –1, +1, +1, +1, –1, –1, –1
� 1Mbit /s uses a binary differential PSK (DBPSK)
� 0 � jω = 0 ; 1 � jω = π
� 2Mbit /s uses a quadrature differential PSK (DQPSK)
� 00 � jω = 0 ; 01 � jω = π/2
� 10 � jω = π ; 11 � jω = 3π/2
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locigno@disi.unitn.it 25
Barker codes
� A sequence of +1 / -1 of length N such that
for all 1
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locigno@disi.unitn.it 26
Tx for 5.5 and 11 Mbit/s
� Uses a complex modulation technique based on HadamardTransforms and known as Complementary Code Keying CCK
� It is a sequence of 8 PSK symbols with the following formula
c = {ej(ϕϕϕϕ1 + ϕϕϕϕ2 + ϕϕϕϕ3 + ϕϕϕϕ4); ej(ϕϕϕϕ1 + ϕϕϕϕ3 + ϕϕϕϕ4); ej(ϕϕϕϕ1 + ϕϕϕϕ2 + ϕϕϕϕ4); –ej(ϕϕϕϕ1 + ϕϕϕϕ4); ej(ϕϕϕϕ1 + ϕϕϕϕ2 + ϕϕϕϕ3); ej(ϕϕϕϕ1 + ϕϕϕϕ3); –ej(ϕϕϕϕ1 + ϕϕϕϕ2); jϕϕϕϕ1 }
ϕϕϕϕi are defined differently for 5.5 and 11 Mbit/s
� The formula defines 8 different complex symbols at 11 Mchip/s
� At 11 Mbit/s 1 bit is mapped on 1 chip, at 5.5 the mapping is 1�2
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locigno@disi.unitn.it 27
Tx for 5.5 and 11 Mbit/s
� In 5.5
� ϕ1 and ϕ3 do not carry information
� 4 bits are pairwise DQPSK encoded on ϕ2 and ϕ4
� In 11
� 8 bits are pairwise DQPSK encoded on ϕ1, ϕ2, ϕ3 and ϕ4
� The resulting signal is a complex PSK modulation over single chips with correlated evolution over the CCK codes
� In practice there are 256 (28) possible codewords but only 32 (5.5 Mbit/s) or 64 (11 Mbit/s) are used
� robustness to fading
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locigno@disi.unitn.it 28
Hadamard Encoding
� We can view them as extension to multiple dimensions of Barker codes
� A broad set of transformation techniques used in many fields
� The base for the MPEG video encoding
� Generalization of Fourier transforms
� Quantum Computing
� …
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locigno@disi.unitn.it 29
Transmission Power Mask
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locigno@disi.unitn.it 30
802.11a OFDM PHY
� 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s
� 6, 12, 24 mandatory
� 52 subcarriers over 20 MHz, 312.5 kHz apart
� Adaptive BPSK, QPSK, 16-QAM, 64-QAM
� OFDM symbol duration 4 µs
� Provides also “halfed” and “quarter” over 10 and 5 MHz by doubling (X 4) the OFDM symbol time
� Convolutional encoding with different rates for error protection
� Encoding is embedded within the OFDM MoDem
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locigno@disi.unitn.it 31
OFDM PPDU
� PLPC is 12 OFDM symbols corresponding to
� Rate defines the DATA rate
� Service is always 0 and enables scrambling synchronization
� SIGNAL is protected with a r=1/2 convolutional code
16
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locigno@disi.unitn.it 32
Sample 16-QAM with gray bit encoding
� Adjacent symbols differs by one bit only
� Makes multi-bit errors less probable
� Associated with interleaving and convolutional encoding greatly reduces BER and hence FER
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locigno@disi.unitn.it 33
Data rates, Slot time and BW
• 802.11a achieves data rates
6,9,12,18,24,36,48, and 54 MB/s.
• One OFDM symbol is sent every
4us, of which 0.8µs is the cyclic prefix
(guard time)
BPSK example:
• 250k symbols sent every second.
• One symbol uses 48 data carriers.
• BPSK modulation with a
convolutional code of rate 1/2
48 * 0.5 * 250k = 6 Mb/s
64-QAM example:
• 250ksymbols/s, 48 data carriers.
• 64-QAM modulation = 64 = 26
• a convolutional code of rate 3/4
48 * 0.75 * 250k *6 = 54 Mbit/s
SLOT TIME
• Slot time = RX-to-TX turnaround time +
MAC processing delay + CCA < 9µs
where CCA = clear channel assessment
Typical times:
• RX-to-TX turnaround time < 2µs
• MAC processing delay < 2µs
• CCA < 4µs
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locigno@disi.unitn.it 34
802.11a/g modulations
2242163/4725464-QAM
2521922/3724864-QAM
3361443/4483616-QAM
504961/2482416-QAM
672723/42418QPSK
1008481/22412QPSK
1344363/4129BPSK
2012241/2126BPSK
T1472 B(µs)
Efficiency(bit/sym.)
FECrate
Gross(Mbit/s)
Net(Mbit/s)
Mod.
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locigno@disi.unitn.it 35
Data rates, Slot time and BW
• 802.11a achieves data rates 6,9,12,18,24,36,48, and 54 MB/s.
• One OFDM symbol is sent every 4us, of which 0.8µs is the cyclic prefix.
BPSK example:• 250k symbols sent every second.
• One symbol uses 48 data carriers.• BPSK modulation with a convolutional code of rate one-half.
=> 48 * 0.5 * 250k = 6 Mb/s.
64-QAM example:
• 250ksymbols/s, 48 data carriers.• 64-QAM modulation = 64 = 26 . • a convolutional code of rate 3/4.
=> 48 * 0.75 * 250k *6 = 54 Mb/s.
SLOT TIME
• Slot time = RX-to-TX turnaround time + MAC processing delay + CCA < 9µs.where CCA = clear channel assessment.
Typical times: • RX-to-TX turnaround time < 2µs
• MAC processing delay < 2µs• CCA < 4µs.
Bandwidth
• One OFDM is 20 MHz and inludes 64 carriers:
=> One carrier = 20MHz/64 = 312 kHz.
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locigno@disi.unitn.it 36
Transmission block scheme
� The modulation is done in the digital domain with an IFFT
� Interleaving distributes (at the receiver) evenly errors avoiding bursts
� Convolutional coding corrects most of the “noise” errors
� This justifies the “observation” that modern 802.11 tends to have an on-off behavior
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locigno@disi.unitn.it 37
Receiver block scheme
� Channel estimation enables distortion correction
� Viterbi decoding is an ML decoder for convolutional codes
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locigno@disi.unitn.it 38
OFDM transmission power mask
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locigno@disi.unitn.it 39
802.11g – ERP
� Extended Rate PHY (as per clause 19 of the standard!!)
� Defines the use of 802.11a OFDM techniques in the 2.4 GHz band
� Mandates backward compatibility with 802.11b
� Introduces some inefficiency for backward compatibility
� Many PPDU formats
� Long/sort preambles
� All OFDM (pure g) or CCK/DSSS Headers with OFDM PSDU (compatibility mode or b/g)
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